The U.S. commander for northern Iraq called for reinforcements yesterday, citing a rise in sectarian attacks and other violence in his region as Iraqi and American forces focus on securing Baghdad.

Attacks have risen 30 percent recently in Diyala province, a mixed Sunni and Shiite region that extends from Baghdad to the Iranian border, as militia fighters and insurgents have flowed in from Baghdad and the Sunni stronghold of Anbar province, Maj. Gen. Benjamin R. Mixon said yesterday.

Diyala's capital, Baqubah, is a "sectarian fault line," Gen. David H. Petraeus, the U.S. commander in Iraq, said yesterday in Baghdad, calling the city "an area of concern right now."

Meanwhile, some U.S. ground commanders in Baghdad are predicting that U.S. troop levels in the Iraqi capital will have to remain elevated until at least the spring of 2008, in order to secure volatile neighborhoods and to prevent them from being reclaimed by insurgents. The comments are a further indication that Petraeus's counterinsurgency strategy is likely to lead to a longer, larger troop increase than Bush administration officials have predicted.

The Pentagon is deploying five Army brigades to Baghdad. Mixon said the need for more troops extends beyond Iraq's capital.

Mixon, commander of Multi-National Division-North, said that he has already shifted some U.S. troops under his command into Diyala but that he needs more and has asked Petraeus's deputy, Lt. Gen. Raymond T. Odierno, to provide them.

"Could I use more forces in Diyala? No question about it. And I'm in discussions of that with General Odierno as he attempts to balance the requirements in Baghdad," Mixon told Pentagon reporters in a videoconference yesterday from Iraq. He declined to specify the number of troops he wanted.

Petraeus said yesterday at a news conference that "very likely there will be additional forces going" to Diyala.

Insurgents have stepped up sectarian attacks in Diyala and also use the area as a staging ground for strikes in neighboring Baghdad, Mixon said.

"We see the Sunni insurgency trying to desperately gain control of Diyala because it helps in their effort to control Baghdad and to prevent the government of Iraq from succeeding," Mixon said.

For example, he said, a tip from an Iraqi source led to the discovery last week in Diyala of a "huge" cache of materials to make "explosively formed projectiles." The cache, he said, included enough material to make about 130 EFPs -- weapons designed to pierce armor.

Pentagon officials have linked EFPs to Iran. Mixon and another commander said that intelligence sources indicated the materials in the cache, including machine-grade copper disks, were manufactured in Iran.

"Many of those munitions were new. Many of them had serial numbers on them that could be traced directly back to Iran," Mixon said. Still, he emphasized that he had "no information" the weapons were linked to the Iranian government.

Inside the capital, Col. Bryan Roberts, commander of the 1st Cavalry Division's 2nd Brigade, cautioned that securing Baghdad's neighborhoods "is a long-term project; there is no doubt about it."

"We're in it for the long haul. It's not 'one year and out' because we have a new plan," Roberts said in a telephone interview from Baghdad.

Roberts, who leads about 4,000 troops operating in central Baghdad, said it is not possible to assess the progress of the security plan until August, and then it would take "a good six more months to build upon our success and transition to the Iraqi security forces." Two brigades of Iraqi security forces working in his sector are at about 75 percent strength and, while improving, still exhibit sectarian biases that have to be held in check by U.S. training teams, he said.

In Baghdad's East Rasheed district, Roberts said, the brigade has reduced sectarian killings since November, as U.S. and Iraqi troops have established five combat outposts and blocked off neighborhoods.

"One of the weaknesses of previous plans was we did not have the forces to control an area once we cleared it of the enemy, so terrorists, militia, gangs were able to come back in," Roberts said. "So with the additional forces, the concept is, you clear it, you control it, and you retain it."